The present invention relates to a fuel supply pump assembly for a fuel injected internal combustion engine, especially a diesel engine, and, more particularly, to a dual-piston, self-priming fuel pump wherein an outer regulating piston is concentrically disposed about an inner pumping piston to maintain fuel pressure within desired limits at all engine speeds and damping potential pressure spikes, the regulating piston further coacting with the inner piston to pump air in the self-priming mode.